In recent years, the heat dissipation management is still the main issue on the development for packaged high power LEDs under high current driving. For the lateral current conducting structure epitaxied on sapphire substrate, there are drawbacks on the current crowding effect, high series resistance and poor heat dissipation.
Then, the disadvantage of poor heat sink of sapphire can be resolved by other heat dissipation substrate by laser lift-off (LLO) process using a short wavelength excimer source. For example, U.S. Pat. No. 7,384,807 to Yoo disclosed a method of fabricating a vertical structure opto-electronic device which includes fabricating a plurality of vertical structure opto-electronic devices on a crystal substrate, and then removing the substrate using a laser lift-off process. However the electrical and optical properties of LEDs are dependent on crystal quality of the epitaxial layers which are subject to additional chemical process (e.g. etching), mechanical process (e.g. lapping), and laser lift-off process. It may be needed for laser beam to scan an epitaxial wafer more than one time, which decreases the throughput and increases the chance of damaging the epitaxial layer of LEDs. Also, the laser lift-off equipment is expensive.
In order to achieve a high yield of LLO process, it is necessary to separate the island of epi-GaN by chemical or physical etching process to form a heat exhaustion structure (so called “street path”). The street path has a benefit of nitrogen gas release during LLO process to achieve high yield.
However, due to an increased dimension of the epi-GaN island, it is necessary to enlarge width of the street path to avoid adjacent islands from attack by the pressure of nitrogen released from photo-induced decomposition.
Sapphire (Al2O3) is the most popular substrate for epitaxial growth of AlInGaN-based materials in light emitting diodes (LEDs) manufacturing. However, the thermal conductivity of the sapphire substrate is not so good that the blue light emitting diode is limited to a lateral conducting structure with its positive and negative metal electrodes on the same side of the substrate. Thus, the structure substantially shrinks the lighting area. Besides, due to current crowding effect and lateral path, there are inevitable increases in series resistance (Rs) and forward voltage drop (Vf) of the LED device. The huge amount of heat generated decays external quantum efficiency directly.
Substituting a traditional sapphire substrate for a heat dissipation substrate by using excimer laser lift-off (LLO) technology has been used for many years. Although it is commercialized, the manufacturing cost of such LED device is still quite expensive due to its complex processes and low yield. According to the high bandgap energy of sapphire, a short wavelength (<355 nm) excimer laser beam can penetrate the polished sapphire substrate and be absorbed in the interface between sapphire and the epitaxial buffer layer (u-GaN). The absorbed energy is accumulated on the surface of u-GaN and converted to heat energy which allows u-GaN to be gasified and nitrogen gas to be released. Subsequently, the released nitrogen gas may cause damage to neighboring insolated epitaxial elements and the heat dissipation substrate due to the pressure thus formed. Therefore, the peeling of the fragile interface formed by bonding or electroplating technology may produce the issue on reliability, for example, fast degradation of light output power, increase of forward voltage drop after package. Therefore, no matter whether bonding process or plating process is used for manufacturing the heat dissipation substrate to replace the current one, it is necessary to use more complex processes to enhance the bonding strength of the interface between the heat dissipation substrate and the epitaxial element. In order to improve the yield rate of LLO, main isolated street path between adjacent two epitaxial elements is necessarily enlarged, which causes low production in a 2-inch wafer and indirectly increases the manufacturing cost.